The founder effect happens when a small group of individuals leaves a large population and starts a new population somewhere else, for example by colonising an island.The founders carry only a small, random sample of the alleles in the original gene pool. By chance, the allele frequencies in this group are likely to be different from those in the parent population. Some alleles may be missing altogether, and a rare allele may be much more common in the founders than it was originally.The new population grows from these founders, so its gene pool reflects their alleles, not those of the original population. If the new population stays isolated, these differences persist, and it has lower genetic diversity than the parent population.
Unlike a bottleneck, the size of the original population is not reduced: the founder effect is about a small group separating and starting a new population. Both are cases of genetic drift having a large effect because the population is small.Example. The Old Order Amish community in Pennsylvania, USA, was founded by a small number of families who emigrated from Europe in the 18th century and mostly married within the community. Ellis-van Creveld syndrome, a recessive condition that causes short stature and extra fingers (polydactyly), is far more common among them than in the general population. One of the founders is thought to have carried the allele.The founder effect helps explain why island populations often differ from mainland ones.
Unlike a bottleneck, the size of the original population is not reduced: the founder effect is about a small group separating and starting a new population. Both are cases of genetic drift having a large effect because the population is small.Example. The Old Order Amish community in Pennsylvania, USA, was founded by a small number of families who emigrated from Europe in the 18th century and mostly married within the community. Ellis-van Creveld syndrome, a recessive condition that causes short stature and extra fingers (polydactyly), is far more common among them than in the general population. One of the founders is thought to have carried the allele.The founder effect helps explain why island populations often differ from mainland ones.